Medical devices for treating cardiac dysrhythiams, such as anti-tachycardia pacemakers and implantable cardioverter defibrillator (ICD) devices are well known. Almost all current ICD devices utilize cardiac detection systems which monitor the heart beat and detect cardiac dysrhythmias based on a tiered therapy concept. Tiered therapy is simply the delivery of a different therapy depending upon how fast the heart is beating when a cardiac dysrhythmia is detected. A tiered therapy cardiac detection system monitors for heart rates that fall in one of several different programmable therapy zones. Each therapy zone is associated with a different tiered therapy to be delivered in response to detection of a cardiac dysrhythmia in that zone. For example, a heart rate in the range of 160 to 200 beats per minute might be classified as indicative of a ventricular tachycardia, in which case the preferred therapy might be anti-tachycardia pacing; whereas a heart rate over more than 240 beats per minute might be indicative of a ventricular fibrillation, in which case the preferred therapy might be delivery of a high voltage defibrillation countershock. A more detailed explanation and comparison of the tiered therapy detection systems of three existing ICD systems (the Medtronic Jewel.RTM., the Ventritex Cadence.RTM. and the CPI Ventak.RTM. PRx.RTM.) can be found in Olson et al., "Properties and Performance of Rate Detection Algorithms in Three Implantable Cardioverter-Defibrillators", IEEE Computers in Cardiology, 0276-6547:65-68 (1994); and Anderson et al., "Performance of Basic Ventricular Tachycardia Detection Algorithms in Implantable Cardioverter Defibrillators", PACE, Vol. 20, Dec. 1997, pp. 2975-2983. Examples of tiered therapy cardiac detection systems can also be found in U.S. Pat. Nos. 4,375,817, 4,830,006, 4,971,058, 5,184,615, 5,318,591, 5,403,355, and 5,458,620.
The advantage of tiered therapy cardiac detection systems over other types of cardiac detection systems is that a tiered therapy cardiac detection system provides for flexibility without undue complexity in terms of how a physician can program the device. While tiered therapy detection systems are preferred because of the ease of physician programming, all of the existing tiered therapy cardiac detection systems are somewhat cumbersome in terms of how the processor inside the device actually implements the process of sorting heart rates into different zones and determining if the programmable criteria for a given zone have been satisfied. Typically, a tiered therapy detection system is implemented by having a microprocessor inside the device perform a counting or "binning" operation for each different zone, i.e., the processor determines the heart rate for a given beat and then, if the heart rate matches a given zone, a marker is stored in or added to a bin or storage location in memory that is associated with that zone. After updating the bins associated with each zone, the processor then checks each bin to see if a separate threshold criteria has been met for that zone, e.g., the processor determines if X of the last Y heart beats have occurred in the zone. It will be apparent that this approach simply requires sufficient memory to create the separate counting bins and the separate criteria for each zone and sufficient processor time to be able to update all of the bins and then check each of the bins against the criteria for each of the zones.
The primary challenge for existing tiered therapy detection systems is resolving detection issues in certain types of erratic dysrhythmias where the heart rate tends to wander across different zones (e.g., the heart rate is over 240 for only a couple of beats, then falls back to 180 for several beats, then advances to over 200 for several more beats). In this situation, it is often difficult to determine which zone is the appropriate therapy zone and when the trigger conditions for delivering therapy have been satisfied for that zone. It is also possible to have an arrhythmia which should be treated aggressively, but which avoids being classified because the trigger conditions, such as the last beat being in that zone, are never quite satisfied for a given zone. In this situation, potentially life-saving therapy might be delayed because of the inability of the therapy detection system to resolve the trigger conditions between different zones. In addition, most tiered therapy detection systems of this type have zone counter reset rules which will reset a zone counter if, for example, four consecutive beats have been detected in a lower zone. To avoid undercounting of beats in a zone, some tiered therapy also utilize an overcounting of lower zones by counting beats occurring in a higher zone as also occurring in a lower zone. Finally, if an arrhythmia terminates spontaneously in that a normal heart beat rate is detected for two or more beats, it is important that the reset conditions for each zone are handled appropriately in the event of a termination of the dysrhythmia. Obviously, there are a myriad of possible interactions and combinations of trigger, reset, overcounting and termination conditions which must be accounted for in implementing a successful and reliable tiered cardiac detection system that utilizes the classic technique of multiple zone binning as described above. For a more detailed explanation of the classic multiple zone binning technique and the potential problems in tachyarrhythmia detection, reference is made to Bach, S. et al., "Tachyarrhythmia Detection," Implantable Cardioverter Defibrillator Therapy: The Engineering Clinical Interface, Chpt. 15, pp. 303-323, eds. Kroll, M. and Lehmann, M. (1996), and Olson, "Safety Margins for Sensing and Detection: Programming Tradeoffs," Id., Chpt. 19, pp. 389-420.
While the existing approach to implementing a tiered therapy cardiac detection system provide very methodical and satisfactory results in terms of useful and reliable detection software for most cardiac dysrhythmias, the programming and processing required by these implementation could be improved and simplified. Additionally, the manner in which tiered therapy cardiac detection systems handle trigger, reset, overcounting and termination conditions could also be improved and simplified. Accordingly, a tiered therapy cardiac detection system which could be implemented more efficiently would provide benefits in terms of quicker response times, more accurate handling of ending, overcounting and reset conditions and reduced power consumption by the microprocessor of an implantable medical device.